1. Field of the Invention
The present invention relates to a semiconductor device in which a flexible film wiring board is connected to a semiconductor element substrate, a process of producing the semiconductor device, and an ink jet recording head using the semiconductor device.
2. Related Background Art
A semiconductor device has been hitherto known which uses a flexible film wiring board such as a TAB (Tape Automated Bonding) or FPC (Flexible Printed Circuit) in order to realize a high integration as disclosed in Japanese Patent Application Laid-Open No. 2003-7765.
FIGS. 7A and 7B are views illustrating a semiconductor device using such a flexible film wiring board. FIG. 7A is a plan view of a main part and FIG. 7B is a sectional view taken along line 7B—7B in FIG. 7A.
In FIGS. 7A and 7B, reference numeral 101 denotes a semiconductor element substrate made of silicon or the like and 102 denotes a flexible film wiring board on which an inner lead 105 serving as an electric wiring pattern in the board is formed. A rectangular device hole 103 for fixing the semiconductor element substrate 101 is formed on the flexible film wiring board 102. Moreover, a flat base film 104 made of an insulating resin such as polyimide is formed on the upper surface of the flexible film wiring board 102. The inner lead 105 is obtained by adhering a metallic foil made of a conductive material such as a copper foil to the lower surface of the base film 104 and patterning the inner lead 105 in a desired shape through the photolithography technique. The lower surface of the patterned inner lead 105 is plated with gold, silver, or solder and moreover, a region of the lower surface from which a metallic face will not be exposed is covered with a resist layer 108 and the like. At this time, a wiring electrode (not illustrated) and an electrode pad (not illustrated) to be bonded to the body are also formed.
Moreover, the inner lead 105 is formed by extending from the flexible film wiring board 102 into the opening of the device hole 103. A plurality of electrode pads 106 are formed on the surface of the semiconductor element substrate 101. The electrode pads 106 are respectively electrically bonded to the front end portion of the inner lead 105 extending into the opening of the device hole 103 through a stud bump 107.
The stud bump 107, which is a metallic protrusion, is previously set on the electrode pad 106, and the inner lead 105 to be bonded is located immediately on the stud bump 107 and the inner lead 105 is bonded to the stud bump 107 by using a bonding tool from above the inner lead 105. Thereby, the inner lead 105 is electrically bonded to the electrode pad 106.
At this time, the semiconductor element substrate 1 is vacuum-attracted to and fixed on a bonding stage so that a preferable bonding state is obtained. This bonding method is normally referred to as ILB (Inner Lead Bonding).
The ILB method is roughly divided into two types. One of them is a gang bonding method for simultaneously bonding all inner leads 105 to stud bumps 107 by a bonding tool for each semiconductor substrate 101. The other of them is a single point bonding method for independently and selectively bonding the inner leads 105 to the stud bumps 107 one by one.
However, in both the gang bonding method and single point bonding method, the inner leads 105 are bonded to the stud bumps 107 while they are heated at a high temperature. To bond the inner lead 105 whose surface is plated with gold to the stud bump 107 of a gold pole by the gang bonding method, it is necessary to heat a bonding tool up to about 500° C. In the case of the single point bonding method, it is necessary to effect heating up to about 200° C.
The thermal expansion coefficients of the base film 104 mainly made of an insulating organic resin and the inner lead 105 mainly made of copper (Cu) are far larger than the thermal expansion coefficient of the semiconductor element substrate 101 made of silicon or the like. Therefore, a displacement occurs in the relative positional relation between the stud bump 107 and inner lead 105 on the semiconductor element substrate 1 in accordance with the thermal expansion of the flexible film wiring board 102.
Therefore, Japanese Patent Application Laid-Open No. 2003-007765 discloses that the array pitch of the inner leads 105 is decided by previously considering the elongation of the base film 104. Moreover, it discloses that a relative shift value of the inner lead 105 from the stud bump 107 is absorbed by making the width of the bonding portion between the inner lead 105 and the stud bump 107 larger than the relative displacement value of the inner lead 105 from the stud bump 107.
However, the number of cases in which plural types of semiconductor element substrates are mounted on one flexible film wiring board has been increased in recent years. For example, there are printing machines using the ink jet system such as shown in FIG. 8 (printer, facsimile machine, copying machine, and multifunction machine thereof). FIG. 9 shows a semiconductor device constituted by mounting semiconductor element substrates on a flexible film wiring board, as mentioned above. In FIG. 9, two device holes 113a and 113b are formed in one flexible film wiring board 102. A semiconductor element substrate (heater board 101a) for a black ink cartridge is mounted in the device hole 113a and a semiconductor element substrate (heater board 101b) for a color ink cartridge is mounted in the device hole 113b. Nozzle members 116a and 116b each having a plurality of discharge openings for discharging an ink are formed on the upper surfaces of the heater boards 101a and 101b. Moreover, the heater boards 101a and 101b are fixed and disposed on a support member 110 and the flexible film wiring board 102 is set by being precisely aligned with the heater boards 101a and 101b. Incidentally, the same reference numerals are employed in FIG. 9 as are employed in FIG. 7 for equivalent elements and their description is omitted.
In FIG. 9, the landing accuracy of ink droplets discharged from the heater boards 101a and 101b are determined by positions of the heater boards 101a and 101b. Therefore, the heater board 101a for black ink cartridge and the heater board 101b for color ink cartridge must be fixed while they are mutually accurately aligned on the support member 110.
However, as described for Japanese Patent Application Laid-Open No. 2003-7765, when heating the inner lead 105 and electrode pad 106 and bonding them to each other and cooling and fixing them on the support member 110, a displacement occurs in the relative position between the flexible film wiring board 2 and the heater boards 101a and 101b. That is, the positions of the flexible film wiring board 2 and the heater boards 101a and 101b are offset with respect to each other due to the difference in thermal expansion coefficient of the flexible film wiring board 2 and the heater boards 101a and 101b during heating and cooling.
Therefore, it has been proposed that the heater boards 101a and 101b are previously accurately positioned on the support member 110 and heated to bond the inner lead 105 and the electrode pad 106 to each other, followed by cooling. In this case, the flexible film wiring board 2 and the heater boards 101a and 101b are heated while they are strongly fixed and are bonded while they are expanded. Thereafter, they are cooled down to ordinary temperature. However, because the inner lead 105 and electrode pad 106 are completely bonded while they are heated, a stress is generated due to the difference in thermal expansion coefficient between the heater boards 101a and 101b and the flexible film wiring board 102 during the cooling step. The stress generated at this time is entirely applied to a bonding portion through the inner lead 105. When the stress exceeds the bonding strength between the electrode pad 106 and the stud bump 107 or the bonding strength between the stud bump 107 and the inner lead 105, peeling off is caused at the boding portion. That is, the reliability of the bonding portion between the inner lead 105 and the electrode pad 106 is deteriorated and in some cases, the portion may be cut or broken. Particularly, in the case where a positional displacement is to be prevented by increasing the width of the inner lead 105 as previously mentioned, the rigidity of the inner lead 105 itself is increased, so that a defect due to the peeling off of the bonding portion will noticeably develop.
Incidentally, the stress generated due to thermal expansion of the flexible film wiring board 102 and heater boards 101a and 101b exists two-dimensionally in the direction of plane shown in FIG. 9.
However, a semiconductor device according to the ordinary TAB method is wound in most cases in a state in which it is mounted on the flexible film wiring board 102 after bonding of the semiconductor element substrate 101 and the flexible film wiring board 102. Therefore, because it is not necessary to fix the semiconductor element substrate 101 to a support member, the above problem does not occur. However, in the case of a semiconductor device for which a high positional accuracy is requested as is the case with an ink jet printing machine, it is necessary to previously fix the semiconductor element substrate 101 to the support member 110, so that the above-mentioned problem will occur.